Question

I am trying to fill my classes with different code depending on template arguments but get a compilation error. My code is like this:

#include <iostream>
#include <string>

struct EmptyType {  };

template<class  arg1=EmptyType, class arg2=EmptyType, class arg3=EmptyType>
class my_class
{
        my_class(){
                std::cout << 3 << std::endl;
        }
    // FILL_MY_CLASS_DEFINE(3)
};
template<class  arg1, class arg2>
class my_class<arg1,arg2,EmptyType>
{
        my_class(){
                std::cout << 2 << std::endl;
        }
    // FILL_MY_CLASS_DEFINE(2)
};
template<class  arg1>
class my_class<arg1,EmptyType,EmptyType>
{
        my_class(){
                std::cout << 1 << std::endl;
        }
};
template<>
class my_class<EmptyType,EmptyType,EmptyType>
{
    // FILL_MY_CLASS_DEFINE(0)
};

int main(int argc, const char *argv[])
{
    my_class<std::string, double, int> a;
    my_class<std::string, int> b;
    my_class<void> c;
        //my_class d;

    return 0;
}

I get lots of errors:

prog.cpp: In function ‘int main(int, const char**)’:
prog.cpp:9: error: ‘my_class<arg1, arg2, arg3>::my_class() [with arg1 = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, arg2 = double, arg3 = int]’ is private
prog.cpp:38: error: within this context
prog.cpp:17: error: ‘my_class<arg1, arg2, EmptyType>::my_class() [with arg1 = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, arg2 = int]’ is private
prog.cpp:39: error: within this context
prog.cpp:25: error: ‘my_class<arg1, EmptyType, EmptyType>::my_class() [with arg1 = void]’ is private
prog.cpp:40: error: within this context

Live code here. So I wonder: is it possible to have different code inside a class when emulating Variadic templates with C++03?

Answer 1

You need to make the constructors of your class public before they can be instantiated.